refactor: add explicit initialisation of the ecc library

[motorina0]

Followup PR of the discussion here: #4 (comment)

Yes. I would prefer something explicit as in the code snippet below for several reasons:

• if the eccLib is invalid, then it will fail at the initEccLib(...) invocation place (which can make it easy to investigate/debug), instead of failing inside p2tr or Psbt
• if p2tr is used without an eccLib then the error message can clearly state that: "No ECC Library provided. You must call initEccLib(...) with a valid TinySecp256k1Interface instance"
  • side note: rename TinySecp256k1Interface to Secp256k1Interface?
• it makes the code-base a little easier to work with (no need to pass eccLib around)
• it makes the public API a little simpler (no eccLib option for p2tr and Psbt)
• calling initEccLib(...) is optional (only required if p2tr is used)
• calling initEccLib(...) multiple times with the same TinySecp256k1Interface instance is a noop

// client code
import { payments, address, initEccLib } from 'bitcoinjs-lib'

initEccLib(clientEccLib)